This invention relates to avalanche photodiodes, and more particularly, to an improved design for avalanche photodiodes for use in high-speed or high bit rate applications.
A need for greater cross-sectional bandwidth in fiber optic network links is widely recognized. The volume of data transmissions has seen a dramatic increase in the last decade. This trend is expected to grow exponentially in the near future. As a result, there exists a need for deploying an infrastructure capable of handling this increased volume and for improvements in system performance.
Fiber optics have gained prominence in telecommunications, instrumentation, cable TV, network, and data transmission and distribution. Major application of fiber optics has been, and will continue to be, in the area of telecommunications. There has been a significant change-over from wires and co-axial cables to optical fibers for telecommunication systems and information services. This change is dictated by the benefits of improved technology as well as economics. Increasing cost and demand for high data rate or large bandwidth per transmission channels and the lack of available space in already congested conduits in every metropolitan area are a key reason in the change over from the wires to fiber optics. Additionally, fiber optical devices interface well with digital data processing equipment, and their technology is compatible with modem microelectronic technology.
In order to avoid the expensive alternative of installing more optical fiber, system designers have traditionally leveraged two technological advances: higher bit transmission rates (time-division multiplexing) and increased number of wavelength channels (wavelength-division multiplexing). The challenge of developing component-level devices with adequately enhanced performance has been addressed by component manufacturers with some progress having been made in a number of areas. A particular example is the development of high-sensitivity optical receivers for performing optical-to-electrical signal conversion at data rates in excess of 10 gigabytes per second (Gb/s).
A canonical fiber optic receiver, such as the one illustrated in FIG. 6, for long-wavelength signals (e.g., 1310 or 1550 nm) contains an InGaAs/InP (Indium-Galium-Arsenide/Indium-Phosphide) photodiode 610 and a trans-impedance amplifier (TIA) 620 as well as post-amplification, clock and data recovery circuitry 630 and a demultiplexer 640 for the high-frequency bit stream. The sensitivity of a receiver, defined as the lowest signal power detected with an acceptably low bit error rate, depends on the front-end elements. There are two options for the photodiode element: a standard p-i-n diode structure, and an avalanche photodiode (APD).
The function of the photo detector in a fiber optic communication link is to convert optical power into electrical response. The most common detector used in fiber applications is the photodiode, which acts as a converter of optical power to electrical current. The type of semiconductor photodiode commonly used for fiber optics application has a reverse bias p-n junction.
As stated above, the two most commonly used photo detectors are the p-i-n photodiode (positive/intrinsic/negative type conductivity) and the avalanche photodiode (APD). Both types of photodiodes are instantaneous photon-to-electron converters where absorbed photons generate hole-electron pairs to produce an electric current. The p-i-n (or pin) and avalanche photodiodes are actually modified p-n junction devices with additional layers at differing doping levels that produce either more efficient quantum conversion or avalanche gain through ionization. A photon is absorbed in a relatively high E (electric) field region, where an electron-hole pair is created. This will produce current in the detector circuit.
In order to obtain high quantum efficiency, the p-i-n photodiode provides absorption in an InGaAs active region. The photo-current of a p-i-n diode is, however, intrinsically quantum-limited. That is, in a best case scenario, each input photon results in only a single electron-hole pair to contributing to the total photo-current.
An alternative approach to the p-i-n photodiode is one in which higher detector currents are created as a result of the avalanche gain effect, as in the case of an avalanche photodiode (APD). Although the APD requires higher operating voltages which must be compensated for temperature shifts, the internal gain of the APD provides a significant enhancement in receiver sensitivity and can be a key enabler in the manufacturing of high sensitivity optical receivers for high speed applications. APDs exhibit internal gain through avalanche multiplication. In the presence of sufficiently high electric field intensity, an initial photon-induced carrier can seed an avalanche process in which carriers obtain enough energy from the electric field to generate additional carrier pairs through impact ionization. By such an effect, a single photon can give rise to tens or even hundreds of carriers which contribute to the resulting photo current.
The avalanche multiplication process is, however, inherently noisy due to its stochastic nature. Under a specific bias condition, the average multiplication gain M is generally the most likely number of carrier pairs created by a given photon; but, there is also a significant probability that any given photon will give rise to Mxe2x88x921 pairs, M+1 pairs, or some other value within a distribution around M. This xe2x80x9cexcess noisexe2x80x9d contribution (which is gain-dependent) can make the APD much noisier than a p-i-n diode in an absolute sense. The noise generated can be a limiting factor on detectivity. However, the comparative usefulness of these devices must be considered in the context of the entire receiver. As long as the avalanche noise is no greater than the noise from other components in the receiver (such as amplifiers), the APD provides a significant increase in the receiver SNR. The increased SNR is particularly attractive at higher frequencies where increased amplifier noise is unavoidable.
FIG. 1 illustrates an increase in receiver signal-to-noise ratio (SNR) when an APD is used. The SNR is directly related to sensitivity. A fundamental noise limitation of an optical receiver is the noise floor of the amplifier which follows the photo-diode. The signal and noise of an APD with unity gain (M=1) are nominally identical to those of a p-i-n diode, and the SNR under this condition is illustrated along the ordinate (vertical axis) of FIG. 1. An increase in the APD signal can be achieved by increasing the gain (along the horizontal axis). This leads to a direct increase in the receiver SNR as long as the APD noise remains less than the amplifier noise floor. The SNR is maximized at an optimum value of the APD gain for which the APD noise is approximately equal to the amplifier noise. Note in FIG. 1 that this is true even though the APD noise may increase faster with gain than the APD signal does.
For low frequency applications, amplifiers can be designed with extremely low noise. Therefore, in the context of lower frequency digital transmission, the use of an APD may not provide a significant benefit. However, at higher bit rates such as 2.5 and 10 Gb/s for example, the sensitivity enhancement of an APD-based receiver can be considerable in relation to a comparably designed p-i-n-based receiver. For InGaAs/InP APDs which are appropriate for long-wavelength telecommunications signals, APD receivers designed for 2.5 Gb/s signals typically provide at least a 7 dB improvement over p-i-n receivers. For data rates of 10 Gb/s, this improvement is slightly less or, approximately 6 dB.
It is desirable to have higher sensitivity in an optical receiver. One measure can be roughly approximated from the cost of optical amplification, which is approximately $1000 per dB. If an APD receiver at 2.5 Gb/s provides an additional 7 dB to a designer""s power budget for a few hundred dollars more than a p-i-n receiver, then the economic advantage of an APD solution over a p-i-n solution becomes more attractive. The APD receiver also offers greater flexibility through its wider dynamic range. However, the complexities of system design at 10 Gb/s introduce additional considerations. For example, in a situation where fiber-induced dispersion poses a greater constraint on signal propagation length than fiber-induced signal attenuation, the higher sensitivity of an APD receiver might seem to offer little advantage. But additional margin in the power budget can provide tolerance for the losses associated with components such as dispersion compensation modules which help to alleviate the fundamental dispersion constraints.
Another context in which the benefit of APD receivers might seem questionable is that of multiple wavelength channels present on a single fiber. If all channels can be simultaneously optically amplified before being split off to p-i-n receivers, then the cost per dB of optical amplification can be substantially reduced by spreading the cost among all the wavelength channels. APD receivers will remain relevant in this situation as long as the design of the APD chip itself enables future costs which do not compare too unfavorably with those of p-i-n diodes. Furthermore, this realization of point-to-point multi-channel transport may have more limited scope in the near future. There is a much greater desire for flexible networks in which wavelength channels serve as the coarse-grained units to be independently added, dropped, and otherwise manipulated. In this scenario, the additional sensitivity of APD-based receivers will provide inexpensive power budget margin for the many network branches which will are likely to be introduced to handle individual wavelengths.
Despite the utility of the APD-based receiver, the design of a high performance, reliable and easily manufactured APD for use at high bit rates poses a challenge. The desire to detect wavelengths in the 1300 nm to 1600 nm range dictates the use of an In0.53Ga0.47As absorption layer grown lattice-matched to an InP substrate, as in standard telecommunications p-i-n diodes. The need to achieve avalanche multiplication requires the presence of sufficiently high electric fields, but the narrow bandgap InGaAs (bandgap xcex5gxcx9c0.75 eV) suffers from intolerably large tunneling leakage currents at electric fields lower than those required to exhibit significant avalanche multiplication in this material.
APD design is complicated by additional factors such as the difficulty of controlling premature avalanche breakdown at the edge of the device. The geometry of planar diffused junctions includes inherent curvature at the junction periphery. This curvature causes locally enhanced electric fields, and the consequent enhanced avalanche at the junction periphery leads to an undesirable non-uniformity in the multiplication profile across the device.
What is desired, therefore, is a device which minimizes noise while maximizing sensitivity and increasing bandwidth for high speed applications.
Accordingly, an object of the present invention is to design a device which minimizes noise.
Another object of the present invention is to design a device which maximizes sensitivity.
A further object of the present invention is to design a device which has an increased bandwidth for high speed applications.
An additional object of the present is to design a device which optimizes the combination of noise, sensitivity and bandwidth.
These and other objects of the present invention are achieved by a separate absorption and multiplication (SAM) avalanche photodiode (APD) having a double diffusion performed on the multiplication layer. In other embodiments the diffusion may be combined with a simultaneously diffused floating guard rings to reduce edge breakdown and increase the bandwidth for high bit rate or high speed applications.